A ball screw having balls movably deposited between a screw shaft and a nut has been in the actual use in various fields such as a positioning mechanism of a machine tool, a vehicle steering, a guide device, a motion screw and the like, since the ball screw enables reduction of a friction coefficient in rotating the screw shaft relative to the nut, as compared with a sliding contact-type screw. In the ball screw, the screw shaft has a spiral ball rolling groove formed thereon and the nut also has a ball rolling groove formed thereon. The ball rolling groove of the screw shaft and the ball rolling groove of the nut are aligned to form a path, in which a plurality of balls is arranged. In the nut, there is formed a circulation path for circulating the balls.
Meanwhile, recently in order to increase the allowable load, there is devised a roller screw using rollers as rolling elements, instead of balls, though the roller screw is not yet in the actual use. In this roller screw, the screw shaft has, instead of the ball rolling groove, a roller rolling groove formed thereon for rolling the rollers, and the nut also has a roller rolling groove formed thereon for rolling the rollers. As each of the rollers is in line contact with the roller rolling grooves, the allowable load of the roller screw can be made higher than that of the ball screw in which each ball is in point contact with the ball rolling grooves.
In use of a screw device, there is a case where the screw device bears a heavy load in one direction along the axis of the screw shaft and only as light a load as self weight in the opposite direction. For example, in an injection molding machine, when liquid (fluidized) resin is injected into a mold with any pressing device, a load of tens of tons is applied to a screw device in the axial direction. When the pressing device is pulled back, as small a load as the pressing device is only applied to the screw device. Also in a press machine, when a mold is pressed with high pressure against materials, a heavy load is applied to a screw device in the axial direction. However, when the mold is separated away from the materials, as small a load as the weight of the mold is only applied to the screw device.
When the screw device is used as one bearing a heavy load only in one direction along the axis of the screw shaft, the screw device cannot be a ball screw because in the ball screw, each ball is in point contact with the ball rolling grooves of the screw shaft and the nut and this point contact makes it difficult to increase the allowable load of the ball screw. On the other hand, the allowable load of the roller screw is allowed to be increased unlike the ball screw, and the roller screw is suitable for use as the screw device bearing a heavy load in one direction along the axis of the screw shaft.
There is known a screw device using both rollers and balls as rolling elements, in which two grooves are formed on each of the screw shaft and the nut, one groove for receiving rollers and the other groove, adjacent to the one groove, for receiving balls (see Patent document 1, paragraphs [0030]-[0033]). In this screw device, the rollers arranged in the one groove are subjected to a heavy “outward” load and the balls arranged in the other groove are subjected to a low “return” load.    [Patent document 1] Japanese Patent Laid-open Publication No. 2004-138136